I think one factor they forgot to add is culture.

When you're firmly entrenched in a specific cultural background, often it can be difficult to relate to someone who grew up in a different culture.

That being said, I'm white, married to a Caribbean woman (who herself is mixed though very dark skinned). Her family still drives me insane though. To say there are cultural differences would be a major understatement... And things I consider normal behaviour are often perplexing to them as well.